\version "2.22.0"

\language english

questionMark = \markup \general-align #Y #CENTER {
   \fontsize #7 \with-color "#6A00F4" "?"
}

#(define (get-text grob)
   (ly:grob-property grob 'text))

#(define (has-text? grob)
   (not (null? (get-text grob))))

#(define (alternate-if-text sym alt)
   (grob-transformer sym
      (lambda (grob orig)
        (if (has-text? grob)
            (if (procedure? alt) (alt grob) alt)
            orig))))

\new Staff \with {
   \omit TimeSignature
   \override NoteHead.stencil = #(alternate-if-text 'stencil ly:text-interface::print)
   \override NoteHead.Y-offset = #(alternate-if-text 'Y-offset 0)
}
{
   \cadenzaOn
   \key c \major
   \clef bass
   f1 g
   \once \override NoteHead.text = \questionMark
   a
   bf c'
   \once \override NoteHead.text = \questionMark
   d'
   \bar "||"
}